1. Field of the Invention
This invention relates generally to the field of infusion pumps. More specifically, this invention relates to an improved device and method for administering a series of controlled-volume dosages of fluid, by themselves or in supplement to a continuous infusion of fluid to deliver a liquid medication to a patient.
2. Prior Art
Infusion pumps have been widely used for many years to administer medications and other fluids to patients. Conventional disposable (single-use) infusion pumps administer a substantially continuous flow of fluid. Examples of such infusion pumps include spring-type and vacuum-type syringe pumps, and balloon-type pumps. Conventional electronic, multiple-use infusion pumps may be programmed to provide a variety of flow regimes such as continuous flow, intermittent flow, and variable flow profiles combining the two. Examples of such infusion pumps include peristaltic pumps, screw-driven syringe pumps, and diaphragm pumps.
In certain applications, infusion of a series of discrete dosages, either alone or in conjunction with a continuous flow, has proven to be beneficial. One such example is in treatment of infections and other medical ailments, where standard clinical practice is to administer a series of dosages over a period of time, each dosage in the series being of controlled volume and infused at a controlled rate of flow. These dosages may be administered alone, with no infusion occurring in the time between dosages, or may be administered with a continuous “keep vein open” or “KVO” flow between dosages to maintain patency of the infusion catheter. Depending on the therapy and drug concentration being used, the size of the controlled-volume dosage may vary from a relatively small dosage of a few cc's or less to a relatively large dosage of 25 to 100 cc's or more.
The problem with conventional electronic pumps used in this application is that electronic pumps tend to be relatively expensive, complex to use and maintain, and inconvenient for use in alternate care sites such as the patient's home. The problem with conventional disposable pumps used in this application is that the pumps are designed to dispense a single dose of medication, and cannot be reused for subsequent dosages without risk of contamination. This requires extra effort by the healthcare provider to prepare multiple pumps, and entails additional expense to purchase multiple pumps.
Some disposable pumps are equipped to provide a series of small dosages, but the size of the dosage is limited to 0.5, 1 or 2 cc's. These devices do not provide a large enough dosage volume to be used for many applications.
Another such example is infusion of pain control medications, where a “patient-controlled analgesia” (PCA) pump can be used to provide a patient-controlled bolus dosage of medication, selectively administered by the patient as needed. Existing PCA pumps take the form of “bolus-only” devices, where the bolus dosages are administered alone, or “basal-bolus” devices where the bolus dosages are supplementary to a continuous basal flow. The state of the art and generally accepted clinical practice requires that a PCA pump have a safety feature that limits the infusion rate to a safe dosage, should the patient attempt to continually administer bolus dosages at a rate that would exceed a safe level of medication intake.
Currently available electronic PCA pumps generally provide the necessary performance, including the ability to program the bolus infusion rate such that the bolus dosage is administered over a longer period of time if desired. However, these electronic pumps tend to be relatively expensive, complex to use and maintain, and inconvenient for use in alternate care sites such as the patient's home.
There are a limited number of available options in disposable PCA pumps that meet this requirement, and their typical function is as follows:                The device provides a medication storage reservoir and a separate bolus dosage reservoir.        The device provides a flowrate-controlling flow restrictor element that limits the rate at which fluid can flow from the medication storage reservoir into the bolus dosage reservoir; this provides the safety mechanism to limit the maximum infusion rate regardless of how often the user attempts to administer a bolus.        The device provides a mechanism whereby the user expels the fluid (the bolus dose) in the bolus dosage reservoir; the typical mechanism is a push button or lever than transmits force from the patient's finger or thumb to compress the bolus dosage reservoir, thereby administering the bolus fluid at a rapid infusion rate.        If the device is a basal-bolus model, it provides a second flowrate-controlling flow restrictor element that limits the speed with which fluid can flow from the medication storage reservoir directly to the patient. This basal flow is typically a parallel flow path that bypasses the bolus dosage reservoir.        
One problem with currently available disposable PCA devices is that they are not well suited for large bolus dosage volumes. Typical disposable PCA devices have a 0.5, 1, or 2 cc bolus dosage volume. Larger bolus dosage volumes of 5, 10, or more cc's have been shown to be clinically efficacious, but impractical with currently-available PCA devices.
Manual force from the patient is required to administer the bolus dosage, and larger dosage volumes require greater manual effort; the manual effort that would be required to administer a large bolus dosage can be a burden on patients in a weakened state. Because only the force of the patient's finger or thumb is flushing the bolus dosage out, existing devices require the patient to maintain the manual effort until the dosage is completely delivered. With a large volume dosage, it may take an extended period of time (several minutes to an hour or longer) for the dosage reservoir to empty, and it is not practical for a patient to maintain finger pressure for such an extended period of time.
The practical size of the bolus dosage is also limited by the fact that the bolus is infused over a short period of time (from a few seconds or less up to several minutes), and the amount of fluid the body can absorb in such a short time is very limited. For example, clinicians treating post-operative pain following orthopedic surgery with a PCA infusion of local anesthetic agent into the surgical site have observed that even a 5-cc bolus dosage often leaks out of the incisions, depriving the patient of the full anesthetic effect of the medication and potentially inhibiting healing of the incision.
Another problem with currently available disposable PCA devices is that they have a bolus reservoir that fills slowly without any patient input. The problem with this is that if a patient does not need a bolus, the unused medication in the bolus reservoir is wasted. With expensive medications, this waste is not economical, especially with large bolus sizes.
Another problem with those currently available disposable PCA devices that provide basal-bolus infusion is that they have two parallel flow paths, each with their own flow restrictor, and a valve is required immediately downstream of the bolus reservoir. The use of two flow restrictors and the valve add cost and complexity to the mechanism. Also, in devices utilizing a passive check valve (which requires a “cracking pressure” that is somewhat higher than the medication reservoir pressure) the patient has to apply significant additional force to the bolus mechanism in order to open the valve to deliver the bolus.
Another problem with currently available disposable PCA devices is that by placing the flow restrictors proximal to the bolus reservoir, the fluid path volume distal to the flow restrictors is relatively large. Since all segments of the fluid path that are distal to the flow restrictor are primed at the restricted flow rate, these devices take a long time to prime (often in excess of 30-60 minutes). This long priming time is inconvenient for the clinicians setting up the device, and is not a cost-effective use of nursing time (especially if the device is being used in an operating room, where wasted setup time can results in hundreds of dollars worth of lost productivity in room usage).
As previously mentioned, these devices have two parallel flow paths, each with their own flow restrictor. A precision flow restrictor is often the costliest component of the device. A device that requires two flow restrictors for two distinct flow rates may be significantly more costly that a device that needs only one flow restrictor to achieve two distinct flow rates, such as the device described herein.
There exists a demonstrated need for an infusion device that is capable of administering a series of controlled-volume dosages of fluid, and offers the following features and benefits:                The dosage reservoir is able to accommodate a relatively large controlled-volume dosage of 5 to 10 cc's or more, or an even larger dosage volume of 25 to 100 cc's or more, and the device infuses the dosage at a controlled rate over an extended period of time;        The dosage reservoir does not fill with medication unless the user activates the dosage, so that medication waste is minimized;        The device minimizes the number and complexity of components, especially expensive components such as flow restrictors, to keep the cost as low as possible;        The device is easy to setup and priming time is minimized; and        The device is easy for the patient to use, with actuation forces minimized and the need to apply force for an extended period of time eliminated.        